1. Field
Embodiments of the invention relate to a magnetic recording medium. More specifically, embodiments of the invention relate to a magnetic recording medium having a protective layer which is thinner and has improved corrosion resistance. Embodiments of invention also relate to a method of manufacturing the magnetic recording medium.
2. Description of the Related Art
The recording capacities of hard disk drives continue to increase ceaselessly, and the recording densities of the magnetic recording media mounted in such drives also continue to rise. As a result of such higher magnetic recording densities, demands have arisen for the read/write head to be brought closer to the recording medium surface, that is, for the distance (magnetic spacing) between the head and the magnetic layer of the magnetic recording medium to be reduced, in order to write to smaller magnetization regions with a higher SN ratio. As one means of reducing the magnetic spacing, reduction of the thickness of the protective layer formed on the magnetic layer, and of the lubricating layer formed thereupon, has been sought.
A protective layer serves to protect the magnetic layer, and must have excellent corrosion resistance, in order that the magnetic layer, comprising metal material, does not react with water or corrosive gas in the atmosphere and corrode. In addition, the protective layer must be durable with respect to head flight, in order that the magnetic layer does not sustain damage even when a head flying at a high relative velocity over the magnetic recording medium with a slight magnetic spacing makes contact with the magnetic recording medium due to some problem. In general, from the standpoints of durability and corrosion resistance, a diamond-like carbon (DLC) film, in which carbon atoms are strongly bonded with finer texture, is used as the protective layer.
Further, a lubricating layer is a layer of a lubricant, formed extremely thinly on the protective layer such that the head can fly smoothly and stably over the magnetic recording medium, and is the uppermost layer of the magnetic recording medium.
In order to improve corrosion resistance, when forming a DLC protective layer, a method of mixing a hydrocarbon gas and hydrogen gas and applying a substrate bias voltage (Japanese Patent Application Laid-open No. 2006-114182), a method of plasma treatment of the surface of the magnetic layer (Japanese Patent Application Laid-open No. 2006-127619), a method of modifying a bias application method during protective film fabrication (Japanese Patent Application Laid-open No. 2007-46115), and a method of regulating the hydrogen content of the protective film (Japanese Patent Application Laid-open No. 2008-123646), have been disclosed. However, in the cases of all of these protective layers, the magnetic layer is extremely easily corroded, so that from the standpoint of corrosion prevention, a protective layer with a film thickness of approximately 5 nm or greater has been required.
Further, the thinner the protective layer is made, the greater is the proportion of strain in the protective layer due to differences in thermal expansion coefficients and other physical properties of the magnetic layer and the protective layer, due to interface mismatch effects, and similar, so that durability is degraded. In Japanese Patent Application Laid-open No. H9-138943, a buffer layer which effectively alleviates such strain is provided between the protective layer and the magnetic layer, to enable a thinner protective layer.
Design guidelines for the hard disk drives of coming products have been described, and a recording density of 1 Tb/in2 and magnetic spacing of 6.5 nm have been proposed. Upon considering the breakdown of the magnetic spacing, the magnetic head's protective layer thickness is approximately 2 nm, the spacing between the uppermost face of the magnetic head and the uppermost face of the magnetic recording medium (the head flying height) is approximately 2 nm, and the lubricating layer of the magnetic recording medium is approximately 0.5 to 1 nm, and hence the thickness of the protective layer of the magnetic recording medium must be 2 nm or less. That is, a protective layer which is even thinner, and moreover affords excellent durability and corrosion resistance, is sought.
Embodiments of the invention provide a protective layer for a magnetic recording medium, having a thickness of 2.5 nm or less and high corrosion resistance. By using this protective layer, a magnetic recording medium is provided which can accommodate high recording densities exceeding 1 Tb/in2.